This normally happens when a new datastore is added to the storage cluster which is associate to a storage policy for the vCloud Director Organization. The rule-set (storage capability) was not properly set to the datastore before it was added to the datastore cluster.

All the datastores, datastore clusters and policies disappear and you get the following errors:Error on provider VDC:No datastores accessible to VDC

On Provider VDC storage policies tab:This storage policy is not backed by any usable datastores accessible from this VDC.There is no usable datastore accessible from this VDC that supports this storage policy. Ensure that this storage policy is backed by at least one datastore in vCenter Server, this datastore is enabled in vCloud Director, and the datastore(s) are accessible by the hosts in the clusters used by this VDC.

Solution:

Verified that all the datastores in the datastore cluster have the storage capability (Tag) assigned which is associated to the provider VDC.